For obtaining color hard copies there have hitherto been studied color recording techniques employing ink-jet process, electrophotographic process and thermal transfer process. Of these processes, the thermal transfer process is advantageous in that it can be easily operated and maintained, enables the use of a compact-sized device therefor for cost reduction, and can be run at a low cost.
The thermal transfer process is classified into two: one is a heat melt transfer process in which a transfer sheet comprising a support having thereon a heat-fusible ink layer (hereinafter referred to as thermal transfer recording material) is heated by a thermal head to transfer the ink from the heat-fusible ink layer onto an image-receiving sheet for thermal transfer recording and the other is a heat diffusion transfer process (heat sublimate process) in which a transfer sheet comprising a support having thereon a heat-diffusible dye (sublimable dye)-containing ink layer is heated by a thermal head to transfer the heat-diffusible dye from the ink layer onto an image-receiving sheet for thermal transfer recording. Of the above two, the latter is more advantageous for full-color recording because it can vary the transferring amount of the dye according to changes in the thermal energy of the thermal head to thereby enable the control of image gradation.
In the thermal transfer recording in the heat-diffusion transfer process, a dye used in a thermal transfer recording material plays an important role, but has the disadvantage that it cannot attain the image stability that is obtained by a conventional dye; i.e., an image formed by the heat-diffusion transfer process is inferior in the resistance to light and the fixability.
In order to improve these points, JP O.P.I. Nos. 78893/1984, 109394/1984 and 2398/1985 disclose image forming methods for forming a chelated dye image on a thermal transfer recording sheet by using a chelatable heat-diffusible dye.
Although these image forming methods are satisfactory for improving the heat resistance and fixability, the diffusion or chelating of the dye coming at the time of thermal transfer from the thermal transfer recording ink sheet thereof is not sufficient in conventional image-receiving sheets to thus form an image lacking in uniformity.
In addition, because the chelated dye is present in the image-receiving layer surface, the formed image has no sufficient resistance to light to thus make it difficult to retain the initial image quality obtained at the time of image formation. Further, where a large amount of a metallic ion-containing compound is present on the image-receiving layer surface, there occur problems that in the image formation with the heat-diffusible thermal transfer recording ink sheet, the dye of the ink layer and its binder together are liable to be transferred (fused) to thus lower the surface gloss of the image recording surface, or the metallic ion-containing compound, at the time of recording, is transferred counter from the image-receiving layer to the ink sheet side, resulting in loss of the image transfer density.